forced flow vapor generating unit



March 1964 P. H. KOCH 3,125,995

FORCED now VAPOR GENERATING UNIT Filed July 27. 1960 5 Sheets-Sheet 1 F|G.1

K INVENTOR.

95 Paul H. Koch ,March 24, 1964 P. H. KOCH 3,125,995

' FORCED FLOW VAPOR GENERATING UNIT FIG.2

INVENTOR.

Paul H. Koch ATTORNEY March 24, 1964 P. H. KOCH FORCED FLOW VAPOR GENERATING UNIT 5 Sheets-Sheet 3 Filed July 27. 1960 INVENTOR.

Paul H. Koch AT TORNE Y March 24, 1964 KOCH 3,125,995

I FORCED FLOW VAPOR GENERATING UNIT I Filed July 27. 1960 5 Sheets-Sheet 4 Q 4' l l r 905 90B 5 y 1 f 1 .11 1 i "i 1 1 Ai I 11 1 i L; 109 IIMIII ll 4 SECOND INITIAL UP-FLOW TUBES OF FURNACE INVENTOR.

Paul H. Koch ATTORNEY March 24, 1964 P. H. KOCH 3,125,995

FORCED FLOW VAPOR GENERATING UNIT 7 Filed July 27. 1960 5 Sheets-Sheet 5 INVENTOR.

Paul H. Koch ATTORNEY r' 3,125,995 Ice Patented Mar. 24', 1964 3,125,995 FGRCED FLQW VAPUR GENERATING UNIT Paui H. Koch, Akron, ()hie, assignor to The Babcoclr & Wiicox Qompany, New York, N.Y., a corporation of New Eersey Fiied July 27, 196%, flea. No. 45,649 9 Claims. (61. l22406) The present invention relates in general to the construction and operation of a forced flow fluid heating unit and more particularly to improvements in the construction and arrangement of fluid heating circuits especially adapted for use in a forced circulation once-through vapor generating and superheating unit and to contain a sub-critical pressure vaporizable fluid.

The general object of the present invention is the provision of a fluid heating unit of the character described so constructed and arranged as to produce superheated vapor from a vaporizable fluid over a wide range of high pressures and temperatures without use of tempering equipment; to assure an optimum distribtution of fluid to all fluid flow paths; to assure an optimum relation of fluid velocity within the tubes to heat input into the tube walls to effect adequate cooling, thereby maintaining the tube walls at a safe temperature; and to provide a division of the fluid heating surface between the radiant and convection heating sections of the unit whereby the vapor generating surface is mainly confined to the boundary walls of the furnace chamber and a substantial portion of the superheating of the fluid is accomplished in the furnace.

The construction of forced circulation once-through steam generators requires the use of a large number of parallel circuits connected between inlet and outlet headers. One of the fundamental problems involved with such a steam generator is the control of the flow through the various parallel circuits in order that the flow in each circuit will be stable and the enthalpy of the fluid discharged from any individual circuit will be close to the average of that from all circuits, in which case the circuits will be in a balanced flow condition. Unbalanced flow may be caused by unequal heat absorption in parallel circuits due to unsymmetrical arrangement of heating surfaces, slag accumulation, or part-load operation with burners out of service; or may be due to unequal resistances caused by different lengths of circuits. When steam or water, or mixtures thereof, is heated in parallel flow paths provided by the furnace wall tubes or tubular panels disposed in the furnace, unbalanced heat and/ or fluid flow distribution may lead to excessive localized tube metal temperatures and/or to excessive temperature differentials between adjacent furnace wall tubes and, thereby, to undue thermal stresses in the furnace wall-forming components. The problem of unequal fluid flow distribution is accentuated when the fluid supplied to the high heat absorbing parallel flow circuits of the furnace is a mixture of steam and water. Whenever a steam-water mixture must be distributed to many tubes of parallel circuits, the possibility of separation of the steam-water mixture exists. Thus one tube may receive saturated steam and another tube may receive saturated water or any combination of the two components. Such a condition imposes a limit on the rate of heat absorption that the tubes of the parallel flow circuits can tolerate without exceeding allowable metal temperatures or allowable temperature differences between adjacent tubes.

So a further and more specific object of the invention is to so proportion and arrange the parallel flow circuits of the furnace that a substantially homogeneous vaporizable fluid is uniformly distributed to the parallel flow fluid flow paths. This aspect of the invention is embodied 2. in a forced circulation fluid heating unit having a furnace chamber supplied with high temperature heating gases, with each wall of the furnace including a row of upwardly extending tubes with alternate tubes arranged for parallel flow of fluid therethrough relative to each other and supplied at their lower ends with a fluid of substantially the same enthalpy and with the remaining tubes arranged for parallel flow of fluid therethrough relative to each other. Provisions are made for connecting the alternate tubes for series flow of fluid to the remaining tubes and for effecting a substantially uniform distribution of the fluids dis charging from the alternate tubes to the remaining tubes at a substantially uniform enthalpy and in a homogeneous form by means of fluid mixing and distribution elements constructed and arranged in accordance with the invention. These elements comprise a hollow spherical vessel connected by a downfiow conduit disposed externally of the furnace for series flow of fluid from the alternate tubes and by a specially arranged supply tube system for flow of fluid to the lower inlet ends of the remaining tubes.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a partially diagrammatic sectional elevation of a forced circulation one-through steam generating unit constructed and operable in accordance with the invention;

FIG. 2 is a plan section taken along the line 22 of FIG. 1;

FIG. 3 is a plan section taken along the line 3-3 of FIG. 1;

FIG. 4 is a diagrammatic representation of the vaporizable fluid flow path within the steam generator of FIG. 1;

FIG. 5 is a plan section taken along the line 5-5 of FIG. 1;

FIG. 6 is a plan section taken along the line 66 of FIG. 1;

FIG. 7 is a plan section taken along the line 7--7 of FIG. 1;

FIG. 8 is an enlarged representation of the lower portion of the furnace of FIG. 1 taken along the line 8-8 of FIG. 10;

FIG. 9 is a fragmentary front view taken along the line 9-9v of FIG. 10; and

FIG. 10 is a fragmentary plan section taken along the line 10-410 of FIG. 8.

In the drawings the invention has been illustrated as embodied in a top-supported forced flow one-through vapor generating and superheating unit designed for the production of superheated steam at pressures below the critical pressure of 3206 psi, a unit of this general construction being disclosed and claimed in my co-pending application, Serial No. 735,819, filed May 16, 1958. The particular unit illustrated is designed for operation at a pressure of 2450 psig. from one third to full load and, alternatively, for operation at variable pressures from one third load at 1300 p.s.i.g. to 64 percent load at 2450 p.s.1.g.

The main portions of the unit illustrated include an upright furnace chamber 10 of substantially rectangular horizontal cross-section defined by a front wall 12, a rear wall 14, side walls 16 and a roof 18 and having a gas outlet 20 at its upper end opening to a horizontally extending gas pass 22 of rectangular vertical cross-section formed by a floor 23 and extensions of the furnace roof 18 and side walls 16. The gas pass 22 communicates at its rear end with the upper end of an upright gas pas sage 24 of rectangular horizontal cross-section formed by a front wall 26, a rear wall 28, side walls 30 and a roof 32. The furnace it) is divided into a pair of compartments 34, 36 by a vertical partition wall 38, each compartment opening at its upper end to the gas outlet 20. The lower portions of the front and rear walls of the furnace slope inwardly and downwardly and cooperate with the furnace side walls to form a hopper 40' and a rectangular throat passage 42 for discharging ash into an ash pit, not shown.

A secondary superheater 44 is disposed in part in the upper portion of the furnace 10 adjacent the gas outlet 20 thereof, with the remainder occupying the furnace end of the gas pass 22. The secondary superheater comprises two groups of tubes 44A and 44B. Group 44A includes laterally spaced pendantly supported radiant heat absorbing tube platens arranged in vertical planes in the direction of gas flow, with each platen having a multiplicity of nested return bend tubes connected at their opposite ends to a horizontal header 154 having inlet and outlet portions 164A and 164-8 separated by a diaphragm 166. Group 443 comprises pendantly supported vertically disposed nested multiple-looped tubes arranged in laterally spaced panels to define parallel flow paths between headers 164B and a transverse header 170 disposed in a compartment 116, with the tubes being arranged so that the vapor flows in parallel flow heat transfer relation with the gases flowing through the gas pass 22 and then in counter flow heat transfer relation. The portion of the gas pass 22 downstream of the secondary superheater 44 is divided into parallel heating gas passes or sections 46 and 4-8 by a vertical baffle 50; and the upright gas passage 24 is divided into parallel heating passes or sections 52 and 54 by a vertical baffle Scythe passes 52 and 54 being continuations of passes 45 and 48, respectively.

Gas pass 52 is occupied by a convection primary superheater 58, gas pass 46 by an intermediate primary superheater 60, gas pass 54 by a primary reheater 62, and gas pass 48 by a secondary reheater 64-. Primary superheater 8 and primary reheater 62 each comprise horizontally extending nested multi-looped tubes arranged in laterally spaced panels, with the tubes of the primary superheater 58 having their opposite ends connected to a lower inlet header 143 and an upper outlet header 144 and arranged so that vapor flows in counterflow heat transfer relation with the gases flowing through the gas pass 52 and with the tubes of the reheater 62 having their opposite ends connected to a lower inlet header 174 and an upper transverse outlet header 176 and arranged so that the vapor flows in counter flow heat transfer relation with the gasses passing through the parallel gas pass 54. Intermediate primary superheater 60 comprises a group of pendantly supported vertically disposed tubes arranged in laterally spaced panels and having their opposite ends connected to header 144 and a transverse outlet header 14 8 disposed in the compartment M6. The secondary reheater 48 comprises vertically disposed nested multiple-looped tubes having their opposite ends connected to header 176 and a transverse outlet header, not shown, in the compartment 1116. Both of the gas passes 54, 52 are occupied by an economizer 66 disposed downstream gaswise of the primary superheater 58 and the reheater 62. The economizer comprises two sections of horizontally arranged multiple-looped return bend tubes disposed across the path of heating gas flow and connected at their opposite ends to outlet and inlet headers 72 and 74 disposed in a vertical plane common to the wall 56 and extending normal to the Walls 26 and 28. The heat- ,ing gases from the gas passes 52 and 54- respectively flow 4 by sets of dampers 71 in the ducts 67. From the airheater the gases pass to an induced draft fan, and then to a stack.

The lower portion of the compartments 34, 36 of the furnace 10 are fired by horizontally extending burners 68 arranged to direct fuel and air in mixing relationship into the compartments through corresponding burner ports in the boundary walls of the furnace. The front, rear and side walls of the furnace each include two vertically spaced rows of burners symmetrically arranged at opposite sides of the vertical centerline of the corresponding wall, Preheated air is supplied to the burners by a forced draft fan, not shown, which discharges air under pressure through the airheater and a duct 7&9 to a vertically extending wind-box 75 enclosing the burners 68 and the lower portion of the boundary walls of the furnace 10.

Feedwater at a pressure of 3070 p.s.i.g. is supplied by a feed pump, not shown, to the economizer 66 wherein it is partially heated, then flows through a downcomer 76 and supply tubes 77 to the inlet headers for alternate tubes of the radiant heat absorbing fluid heating tubes lining the front, rear and side walls of the furnace 10.

The front, rear and side Walls of the furnace 10 and the side walls of the gas pass 22 are formed in most part by insulation covered fluid heating tubes. In accordance with the present invention, the furnace boundary wall fluid heating surface is so proportioned and arranged that the distribution of flow to all fluid flow paths is substantially uniform; that the maximum temperature differential between adjacent tubes is below a predetermined critical limit, thereby maintaining differential expansion in the walls Within safe limits; that fluid flow unbalances in the tubes are minimized; that the tube surfaces in diflerent zones of heat intensity in the furnace are sufficient in quantity to carry away the heat at a rate adequate to prevent overheating of the tubes; and that the tubes are of suflicient inside diameter along their lengths to provide adequate fluid circulation velocities. Accordingly, each of the boundary walls of the furnace It) is lined by a row of upwardly extending parallel tubes arranged in groups to form coplanar laterally contiguous radiant heat absorbing tubular panels, the front wall 12 having a row of tubes 89, the rear wall 14 including a row of tubes 87, and each side wall having a row of tubes 90. The tubes $0 also line the portion of the side walls of the gas pass 22 opposite the secondary superheater 44. Some of the rear wall tubes 87 have their upper portions bent inwardly and upwardly to form a nose arch 96; then rearwardly and upwardly to form the floor 23 of the gas'pass 22; and then vertically to form part of a screen 98 disposed at the rear end of the gas pass 22. Intermediate portions of some of the tubes of the front, rear and side Walls in the furnace are suitably bent to form the openings or ports for the burners 68. The furnace 10 has its corners beveled to promote heat absorption in the fluid heating tubes thereat, these tubes constituting the end tubes of the front and rear Walls of the furnace.

For the sake of clarity FIG. 4 diagrammatically shows the flow path of the vaporizable fluid through the vapor generator, and particularly illustrates that the fluid flow path through the tubes of each of the upright boundary walls of the furnace is substantially the same. As shown in FIGS. 1, 4 and 6, the front wall 12 includes a multiplicity of initial or first pass upflow tubes SiiA arranged in parallel spaced relation and for parallel flow of fluid therethrough, spaced about a tube diameter apart to provide intertube spaces, and extending throughout the high heat intensity burner zone of the furnace from the level ofthe top of the hopper to a point about half way between the 'top row of burners and the nose arch )6. The tubes 80A have their opposite ends connected to horizontal outlet and inlet headers 82 and 84-, respectively, disposed outside of the wall 12, while the corresponding tubes of each side wall 16 have their opposite ends connected to horizontal outlet and inlet headers 92 and 94, and the corresponding tubes of the rear wall 14 have their lower and upper ends connected to horizontal inlet and outlet headers 88 and 86, respectively.

Since the tube lengths in the burner zone are exposed to gases of high heat intensity than those above and below this zone, their total absorption is higher and the quantity of heating surface presented by the tube lengths in the burner zone must be greater than that above and below the burner zone to carry away the heat at a rate suflicient to prevent overheating of the tubes. Accordingly, the front wall 12, as well as the other furnace boundary walls, also includes a multiplicity of second upflow tubes, designated as tubes 80B ifOl the front wall, extending throughout the height of the furnace, arranged in parallel spaced relation and for parallel flow of fluid therethrough, and disposed in the spaces between and contiguous to the initial upflow tubes, tubes 80A in the case of the front wall, along the height thereof, so that the number of tubes presented to the gases in the zone of high heat intensity is double that above and below this zone, as shown in FIGS. 5-7. Thus the tubes 8313 are spaced about a tube diameter apart and contact the tubes 80A through the height of the burner zone. The same relation applies to the corresponding tubes of the other upright furnace boundary walls. The tubes 803 have their upper and lower ends connected respectively to horizontal outlet and inlet headers 85 and 83 situated outside of the furnace; while the corresponding tubes WB of each side wall 16, as shown in FIGS. 1 and 8, have their opposite ends connected to horizontal inlet and outlet headers 95 and 93, and the corresponding tubes of the rear wall 14 have their lower and upper ends connected to horizontal headers S1 and 89, respectively. The furnace boundary wall fluid heating tube portions in the zone of high heat intensity are covered by metallic casing suitably secured thereto, while the furnace boundary wall tube portions above and below the zone of high heat intensity, these being second up-flow tube portions, have their intertube spaces closed by metallic webs rigidly secured to the tubes.

Water of substantially the same enthalpy and in a sub-cooled condition, that is, at a temperature below the saturation temperature corresponding to the pressure, is supplied in parallel flow relation from the downcomer 76 to the front wall fluid supply headers 84, as well as the corresponding headers 94 and 8 8 of the side walls and rear wall, by the supply tubes 77.

By way of example, and not of limitation, the furnace wall tube portions below the burner zone are As" OD. x .40 ID. on 1 /2 centerlines; the tube portions in the burner zone of high heat intensity are A" OD. x .40 ID. and substantially tangent to each other; and the tube portions above the burner zone are Ma" -O.D. on 1 /2 centerlines and .40" I.D. up to about the level of the nose arch 96 and .55 ID. above the level of the nose arch. The second upflow tubes of the furnace boundary walls are swaged from A1 to A" OD. immediately before entering the burner zone and from to /8" OD. immediately after the burner zone.

Outlet headers 82, 86 and 92 are connected for series flow of the fluid heated in the initial upflow tubes of the furnace boundary walls to a header 1 12A by tubular connectors 100, as shown in FIG. 1. Header 112A constitutes one leg of a continuous collecting header 112 disposed along the periphery of the gas pass 24 at a position intermediate the economizer 66 and the primary superheater S8 and arranged to supply fluid to the tubes forming the baflle walls of the gas pass 22 and the upright gas passage 24 and to the tubes lining the side walls of the gas pass 22 and the boundary walls of the gas passage 24. The portion of each of the side walls of the gas pass 22 downstream of the secondary 'superheater 44 is lined by a row of vertically extending. closely spaced parallel tubes 10 3 extending between a horizontal upper header 1116 and a horizontal lower header 1118 disposed between rear wall '14 and front wall 26, of the upright gas pass 24, with each header 106 having one end connected to a header 1112 extending across the full width of the unit and with each header 108 being connected at one end to the header 112A. The lower portions of the tubes 103 of the side walls cooperate with the rear wall 114, front wall 26 and a floor 114 to form the compartment 116 Battle wall 5% is formed by a row of vertically extending parallel tubes 1118 arranged in contacting relation and extending between a horizontal upper header 121D and a horizontal lower header 122 disposed in the compartment 115, with the outlet header having one end connected to the mid-portion of header E102 and the inlet header 122 connected to the header 112A by tubes 1%.

The front wall 26 of the gas passage 24 is formed by a row of vertically extending spaced parallel tubes 128 having their upper ends connected to a transverse horizontal header 1111 forming a part of the roof of the gas pass 22 and their lower ends connected to the header 112A; their upper portions bent to form, along with the upper portions of the tubes 87, the screen 98; and their intertube spaces at their lower portions closed by metallic Webs. The roof 32 is partially formed by and the rear wall 28 is lined by a row of upwardly extending spaced parallel tubes 1310 having their upper ends connected to the header 101 and their lower ends connected to a leg 112B of the header 112. Each side wall 311 is lined by a row of closely spaced vertically extending parallel tubes 132 extending between an upper horizontal header 134 and a leg 112C of the header 112, with each outlet header 134 being connected at one end to the header 102. Baffle wall 56 is formed in part by a row of vertically extending tangent parallel tubes 138 having their opposite ends connected to upper and lower horizontal headers 14% and 142, respectively, with the outlet header 14 having one end connected to the mid-portion of header 102 and with the inlet header 142 having its opposite ends connected to the mid-portions of the legs 112A and 1123 of the header 112. The remainder of the baflie 56 is formed by a metallic wall, not shown, suitably secured to the header 142 and the economizer headers 72 and 74 and extending downwardly from the bottom of the header 142 to the ducts 67.

The fluids discharged from the side and batfle wall tubes of the gas pass 22 and the gas passage 24 are mixed in passing through the common collecting header 1612, then discharge therefrom to the header 1M by way of tubular connectors 111. Thus the header 112 is connected for parallel upflow of fluid to tubes of the side and baflie walls of the gas pass 22 and the boundary and baflle walls of the upright gas passage 24 and the header 101 is connected to receive the fluids discharging from all of these tubes. The header 1111 is connected for flow of the fluids collected and mixed therein to a transverse horizontal header 113 by a row of spaced parallel tubes 104 lining the roof 13 of the furnace 1t and the gas pass 22.

While the fluid heating surfaces of the unit are proportioned and arranged so that at full load and designed operating pressure the portion of the heated fluid circuit in which the transition of the fluid from a water condition to a steam-Water condition will be located in the second upflow tubes of the furnace, it is expected that under certain load and pressure conditions the transition will take place before the fluid reaches the second upflow tube. Thus the fluid supply system for the second upflow tubes of the furnace must be so constructed and arranged as to inhibit or prevent separation of the steam from the water when the fluid consists of a mixture of both. In addition the system must promote mixing of the fluid streams whether they be in a water or a steamwater condition, as they pass from the initial upflow tubes to the second upflow tubes of the furnace, and thereby provide a substantially uniform fluid enthalpy upon discharge to the second upflow tubes; and must provide uniform distribution of the fluids to the second upflow tubes. Accordingly, the fluids collected in the header 113 are passed without further heating to the fluid supply headers 95, 83 and 81 for the second upflow tubes of the furnace boundary walls by means of fluid mixing and distribution apparatus constructed and arranged in accordance with the invention and shown in FIGS. 8-10. The fluid mixing and distribution apparatus provides, in effect, an interruption in the fluid flow path through the furnace walls at which the fluids discharging from the initial upflow tubes of the furnace and the walls of the convection gas passes are collected, then mixed to neutralize diiferences in amount of heat picked up and then uniformly distributed to the second upflow tubes of the furnace. This apparatus comprises a single upright downcomer 195 of relatively large diameter extending along and outside of the front wall 12 and having its upper inlet end connected to the mid-portion of the header 113 and its lower outlet end connected to and leading vertically downward and radially into the topof a hollow metallic vessel 1W7 of spherical form located subjacent the hopper 40. The vessel 197 is so connected to the fluid supply headers for the second upflow tubes 9GB of the side wall 16 and the corresponding headers of the other furnace boundary walls, that the fluids mixed in the downcomer 105 and the vessel 107 are uniformly distributed to these headers. The fluid supply system for each of the fluid supply headers 83, S1 and 9500mprises one or more supply tubes 109 each opening at one end to the vessel 107 and arranged to discharge fluid in parallel flo-w relation to a group of tubes 115 having their outlet ends connected to the corresponding fluid supply header at uniformly spaced positions along the length thereof. The tubes 10% are equally spaced about the periphery of the vessel 107 at their points of connection thereto and lead radially from the vessel 167 in a common horizontal plane, then extend vertically for distribution of fluid to the tubes 115, with the vertical portion of each tube 199 having its upper end capped by a fluid distribution nipple 117. Tubes 115 of each group lead radially from a corresponding nipple in a common horizontal plane, then extend vertically for connection to one of the fluid supply headers. Outlet headers 93, 85 and 89 of the second upflow tubes of the furnace side, front and rear walls are connected by tubes 1159 for series flow of the vapor-liquid mixtures generated in the second upflow tubes to a collecting header 121 from which the fluids pass through a conduit 123 and branches 123A thereof to the inlet header 143 of the primary superheater 58.

The particular form of the vessel 197 and the routing and arrangement of the conduits leading to and from the vessel assure that a completely homogeneous fluid of uniform enthalpy will be discharged to the second upflow tubes because the fluid streams are intimately mixed as they pass from the initial to the second upflow tubes of the furnace and because separation of steam from Water is inhibited or prevented whenever the fluid is so constituted; and provide a pressure drop of a magnitude suflicient to assure a substantially uniform flow of fluid to the parallel flow second upflow tubes of the furnace. Because of the uniform enthalpy and distribution of the fluid entering the second upflow tubes of the furnace and because the temperature of the fluid is no greater than the saturated temperature as it flows through the second upflow tubes, the maximum differential temperature between adjacent initial and second upflow tubes of the furnace will be maintained well below the limit beyond which undue thermal stresses on the furnace wall-forming components would be expected to occur.

By way of example, and not of limitation, the downcomerlflS is 9" ID. and has a vertical height of about 114 ft.; the vessel 197 is30 LD. and supplies fluid to thirty-six of the tubes 109, With each tube N9 in turn supplying fluid to four of the tubes 115 which distribute fluid to eleven hundred and thirty-eight second upflow tubes of the furnace by way of the inlet headers thereof. In the preferred construction arrangement of the fluid mixing and apparatus, tubes 169 should have at least the equivalent of ten diameters of vertical straight length ahead of the nipples 117 and should have a cross-sectional area suflicient to provide a minimum fluid velocity of 10 ft. per second.

From the header 144 the partly superheated vapor passes through the intermediate primary superheater 6i into outlet header 14-8, then flows through a downcomer 15d and supply tubes 152 to the final primary vapor superheating tubes forming the partition wall 38 of the furnace. The partition wall 38 extends normal to the front and rear walls 12 and 14 of the furnace and is formed by coplanar tubular panels, each panel being formed by vertically extending final primary vapor superheating tubes 154 contacting each other along their lengths and having their opposite ends connected to lower'inlet headers 156 and a common upper outlet header 155, with the panels being arranged so as to provide openings 160 for flow of gases between the compartments 34, 36 of the furnace. Header 155 constitutes one leg of a horizontally arranged cross-shaped header, the other leg 155A extending normal to the leg 155 and parallel to the front wall. Headers 156 are connected to the downcomer by the tubes 152 to provide parallel upflow of vapor through the tubes 154. The vapor thus further superheated passes from the header to the header 155A, and then through tubes 162 to the inlet headers 164A of the secondary superheater 4 3. The vapor receives its final superheating in the secondary superheater 44 and is discharged to outlet header 174 from which it passes through a conduit 172 to the high pressure stage of the vapor turbine.

Thus in operation, as shown in FIG. 4, the high pres sure fluid supplied by the feed pump passes through the economizer 66; then flows upwardly in parallel through the radiant heat absorbing initial upflow tubes of the front, rear and side walls of the furnace to the collecting header 112; then in parallel upflow through the convection heat absorbing fluid heating tubes of the side and bafile walls of the gas pass 22 and the boundary and baflie walls of the gas passage 24; then through the tubes 104 forming the roof of the furnace 1t and the gas pass 22; then flows through the downcomer 105 and vessel 107 for uniform distribution of the fluid to the headers 83, S1, 95; then flows in parallel through the radiant heat absorbing second upflow tubes of the front, rear and side walls of the furnace; then successively passes through the primary superheater 58, the intermediate primary superheater 60, the final primary vapor superheating tubes forming the division wall 38 of the furnace and the secondary superheater 44; and then flows to the high pressure stage of the turbine. Partially expanded steam from the turbine successively passes through the primary reheater 62 and the secondary reheater 64, from which it passes to the turbine for final expansion.

Combustion air and fuel are supplied through the burner ports to the lower portion of each of the compartments 34, 36. The resulting heating gases flow upwardly through the compartments 3.4, 36 and over the radiant heating absorbing portion 44A of the secondary superheater to the gas inlet of the gas pass 22; then flows horizontally through the furnace end of the gas pass 22 in contact with the convection heat absorbing portion 44B of the secondary superheater; and then divides into parallel streams, with one stream successively passing through the gas section 46 and the gas section 52 in contact with the intermediate primary vapor superheating tubes 60 and the primary vapor superheating tubes 58, and the other stream successively passing through the gas section 48 and the gas sec tion 54 in contact with the secondary vapor reheating tubes 64 and the primary vapor reheating tubes 62.

The superheating and reheating surfaces are proportioned and arranged to provide the required final or outlet steam temperatures at full steam load. The proportion of the heating gases flowing through the gas sections 48, 54 and 46, 52 are respectively increased and decreased by means of the dampers 71 as the rate of steam generations decreases to hold the reheater outlet steam temperature constant over a wide range of steam loads, while the final superheater outlet steam temperature is maintained constant by controlling the firing rate of the burners. The vapor generating and superheating surfaces are proportioned and arranged so that the portion of the heated fluid circuit in which the transition of the fluid from a steam-water condition to a steam condition occurs will always be located in the relatively low temperature primary superheater 58 throughout the operating range.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed Without departing from the spirit of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber, a row of upwardly extending laterally spaced initial upflow tubes disposed in said furnace and arranged for parallel flow of fluid therethrough, a row of upwardly extending laterally spaced second upflow tubes disposed in said furnace and arranged for parallel flow of fluid therethrough, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said initial upflow tubes, and means connecting said initial upflow tubes for series flow of fluid to said second upflow tubes and effecting a substantially uniform distribution of the fluid discharging from said initial upflow tubes to said second upflow tubes at a substantially uniform enthalpy, said last named means comprising a spherical vessel, downflow tubular means disposed externally of said furnace and connected for series flow of fluid from said initial upflow tubes and to said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said second upflow tubes.

2. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber, a row of upwardly extending laterally spaced initial upflow tubes disposed in said furnace and arranged for parallel flow of fluid therethrough, a row of upwardly extending laterally spaced second upflow tubes disposed in said furnace and arranged for parallel flow of fluid therethrough, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said initial upflow tubes, and means connecting said initial upflow tubes for series flow of fluid to said second upflow tubes and effecting a substantially uniform distribution of the fluid discharging from said initial upflow tubes to said second upflow tubes at a substantially uniform enthalpy, said last named means comprising a hollow spherical vessel, a downflow conduit disposed externally of said furnace and connected for series flow of fluid from said initial upflow tubes and to the top of said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said second upflow tubes and including a multiplicity of tubes extending radially from said vessel in a common horizontal plane.

3. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber, at least one of said walls including a row of upwardly extending tubes with alternate tubes arranged for parallel flow of fluid therethrough relative to each other and with the remaining tubes arranged for parallel flow of fluid therethrough relative to each other, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said alternate tubes, and means connecting said alternate tubes for series flow of fluid to said remaining tubes and effecting a substantially uniform distribution of the fluids discharging from said alternate tubes to said remaining tubes at a substantially uniform enthalpy, said last named means comprising a spherical vessel, downflow tubular means disposed externally of said furnace and connected for series flow of fluid from said alternate tubes and to said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said remaining tubes.

4. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber, at least one of said walls including a row of upwardly extending laterally spaced tubes with alternate tubes arranged for parallel flow of fluid therethrough relative to each other and with the remaining tubes arranged for parallel flow of fluid therethrough relative to each other, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said alternate tubes, and means connecting said alternate tubes for series flow of fluid to said remaining tubes and effecting a substantially uniform distribution of the fluids discharging from said alternate tubes to said remaining tubes at a substantially uniform enthalpy, said last named means comprising a hollow spherical vessel, a downflow conduit disposed externally of said furnace and connected for series flow of fluid from said alternate tubes and to said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said remaining tubes and including a multiplicity of tubes extending radially from said vessel in a common horizontal plane.

5. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber, at least one of said walls including a row of upwardly extending tubes with alternate tubes arranged for parallel upflow of fluid therethrough relative to each other and with the remaining tubes arranged for parallel upflow of fluid therethrough relative to each other, a plurality of inlet headers each connected to a group of said remaining tubes, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said alternate tubes, and means connecting said alternate tubes for series flow of fluid to said remaining tubes and effecting a substantially uniform distribution of the fluids discharging from said alternate tubes to said remaining tubes at a substantially uniform enthalpy, said last named means comprising a hollow spherical vessel, a vertical downflow conduit disposed externally of said furnace and connected for series flow of fluid from said alternate tubes and to the top of said vessel, a multiplicity of tubes having inlet portions extending radially from said vessel in a common horizontal plane and outlet portions extending vertically upward, a fluid distribution nipple closing the upper end of the outlet portion of each of said last named tubes, and groups of tubes connecting the distribution nipples to the inlet headers of said remaining tubes, with the tubes of each group entering one of said inlet headers at spaced positions along the length thereof and extending radially from one of said nipples in a common horizontal plane.

6. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber and establishing a gas flow zone inter: mediate the height of said chamber and in the vicinity of said burner means of higher heat intensity than exists in the portion of said chamber above and below said zone, at least one of said walls including a row of upwardly extending tubes with alternate tubes arranged for parallel flow of fluid therethrough relative to each other and extending through said zone of high heat intensity and with the remaining tubes arranged for parallel flow of fluid therethrough relative to each other and extending throughout and above and below said zone of high heat intensity, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said alternate tubes, said alternate tubes being arranged in parallel spaced relation to provide intertube spaces, said remaining tubes being disposed in the spaces between and contiguous to said alternate tubes along the height of said zone so that the number of tube legs presented to the gases in the zone of high heat intensity is double that below and above said zone, and means connecting said alternate tubes for series flow of fluid to said remaining tubes and effecting a substantially uniform distribution of the fluid discharging from said alternate tubes to said remaining tubes at a substantially uniform enthalpy, said last named means comprising a spherical vessel, downflow tubular means disposed externally of said furnace and connected for flow of fluid from said alternate tubes and to said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said remaining tubes of said tube panels.

7. In a forced circulation fluid heating unit, walls defining a furnace chamber for the flow of heating gases, burner means supplying high temperature heating gases to said chamber and establishing a gasflow Zone intermediate the height of said chamber and in the vicinity of said burner means of higher heat intensity than exists in the portion of said chamber above and below said zone, at least one of said walls including a row of upwardly extending tubes with alternate tubes arranged for parallel upflow of fluid therethrough relative to each other and extending through said zone of high heat intensity and with the remaining tubes arranged for parallel upflow of fluid therethrough relative to each other and extending throughout and above and below said zone of high heat intensity, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to the lower inlet ends of said alternate tubes, said alternate tubes being arranged in parallel spaced relation to provide intertube spaces, said remaining tubes being disposed in the spaces between and contiguous to said alternate tubes along the height of said zone so that the number of tube legs presented to the gases in the zone of high heat intensity is double that below and above said zone, and means connecting said alternate tubes for series flow of fluid to said remaining tubes and eifecting a substantially uniform distribution of the fluid discharging from said alternate tubes to said remaining tubes at a substantially uniform enthalpy, said last named means comprising a hollow spherical vessel, a downflow conduit disposed externally of said furnace and connected for flow of fluid from said alternate tubes and to the top of said vessel, and means connecting said vessel for flow of fluid to the lower inlet ends of said remaining tubes and including a multiplicity of tubes extending radially from said vessel in a common horizontal plane.

8. A once-through forced circulation vapor generator comprising walls including radiant heat absorbing fluid heating tubes defining an upright furnace chamber having a heating gas outlet, means including convection heat absorbing fluid heating tubes forming a gas pass serially connected to said gas outlet, a bank of vapor superheating tubes positioned in said gas pass in the path of gas flow, means for burning fuel in said furnace chamber, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to alternate tubes of each of said walls, and means for interconnecting said fluid heating and vapor superheating tubes to provide a serial upflow of fluid successively through the alternate radiant heat absorbing fluid heating tubes of said Walls, the convection heat absorbing fluid heating tubes, the remaining radiant heat absorbing fluid heating tubes of said walls, and the bank of vapor superheating tubes.

9. A once-through forced circulation vapor generator comprising walls including upwardly extending radiant heat absorbing fluid heating tubes defining an upright furnace chamber having a heating gas outlet, means including convection heat absorbing fluid heating tubes forming a gas pass serially connected to said gas outlet, a bank of vapor superheating tubes positioned in said gas pass in the path of gas flow, means including a row of vapor superheating tubes dividing said furnace chamber into a pair of gas flow compartments each opening to said gas outlet, means for burning fuel in said furnace chamber, means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation directly from a common source to alternate tubes of each of said walls, and means for interconnecting said fluid heating and vapor superheating tubes to provide a serial upflow of fluid successively through the alternate radiant heat absorbing fluid heating tubes of said walls, the convection heat absorbing fluid heating tubes, the remaining radiant heat absorbing fluid heating tubes of said walls, the vapor superheating tubes dividing said furance chamber, and the bank of vapor superheating tubes.

References Cited in the file of this patent UNITED STATES PATENTS 1,975,096 Fletcher Oct. 2, 1934 2,567,695 Cox Sept. 11, 1951 2,818,837 Frisch Jan. 7, 1958 2,902,982 Rowand et al Sept. 8, 1959 2,962,005 Koch Nov. 29, 1960 3,007,459 Koch Nov. 7, 1961 

1. IN A FORCED CIRCULATION FLUID HEATING UNIT, WALLS DEFINING A FURNACE CHAMBER FOR THE FLOW OF HEATING GASES, BURNER MEANS SUPPLYING HIGH TEMPERATURE HEATING GASES TO SAID CHAMBER, A ROW OF UPWARDLY EXTENDING LATERALLY SPACED INITIAL UPFLOW TUBES DISPOSED IN SAID FURNACE AND ARRANGED FOR PARALLEL FLOW OF FLUID THERETHROUGH, A ROW OF UPWARDLY EXTENDING LATERALLY SPACED SECOND UPFLOW TUBES DISPOSED IN SAID FURNACE AND ARRANGED FOR PARALLEL FLOW OF FLUID THERETHROUGH, MEANS FOR SUPPLYING A VAPORIZABLE FLUID OF SUBSTANTIALLY THE SAME ENTHALPY IN PARALLEL FLOW RELATION DIRECTLY FROM A COMMON SOURCE TO THE LOWER INLET ENDS OF SAID INITIAL UPFLOW TUBES, AND MEANS CONNECTING SAID INITIAL UPFLOW TUBES FOR SERIES FLOW OF FLUID TO SAID SECOND UPFLOW TUBES AND EFFECTING A SUBSTANTIALLY UNIFORM DISTRIBUTION OF THE FLUID DISCHARGING FROM SAID INITIAL UPFLOW TUBES TO SAID SECOND UPFLOW TUBES AT A SUBSTANTIALLY UNIFORM ENTHALPY, SAID LAST NAMED MEANS COMPRISING A SPHERICAL VESSEL, DOWNFLOW TUBULAR MEANS DISPOSED EXTERNALLY OF SAID FURNACE AND CONNECTED FOR SERIES FLOW OF FLUID FROM SAID INITIAL UPFLOW TUBES AND TO SAID VESSEL, AND 